Injury to the nervous system has serious consequences. Following traumatic brain injury and stroke, the normal response of the surrounding brain is to mount a cellular response that includes formation of reactive astrocytes that are believed to be important to “contain” and “clean-up” the injury site. Swelling of neural cells is part of the cytotoxic or cell swelling response that characterizes brain damage in cerebral ischemia and traumatic brain injury, and is a major cause of morbidity and mortality. See, Staub et al., 1993; Kimelberg et al., 1995. A number of mediators have been identified that initiate swelling of neural cells, including elevation of extracellular K+, acidosis, release of neurotransmitters and free fatty acids. See, Kempski et al., 1991; Rutledge and Kimelberg, 1996; Mongin et al., 1999. Cytotoxic edema is a well-recognized phenomenon clinically that causes brain swelling, which worsens outcome and increases morbidity and mortality in brain injury and stroke.
Mechanisms underlying apoptotic death of reactive astrocytes and other cells have been studied. See, Tanaka et al., 2000; Yu et al., 2001. The mechanisms responsible for necrotic cell death of astrocytes, neurons and neural endothelial cells have not been characterized. Apoptotic cell death is preceded by cell shrinkage and net loss of K+. See, Yu et al., 1997; Yu et al., 1999. By contrast, in necrotic cell death, the plasma membrane is ruptured, causing cytosolic contents to be released and thereby triggering tissue inflammation. See, Leist and Nicotera, 1997. Necrotic cell death may be more deleterious to nearby viable tissues, given the secondary inflammatory damage that is initiated.
Necrotic cell death is initiated by osmotic swelling following influx of Na+, the major extracellular osmolyte. In most cell types, accumulation of Na+ intracellularly is regarded as a passive process that does not require activation of specific effectors but that is due instead to defective outward Na′ pumping under conditions of low intracellular adenosine triphosphate concentration ([ATP]i). See, Leist and Nicotera, 1997; Trump et al., 1997. Cell blebbing or swelling, an indication of intracellular Na′ overload, is generally regarded as an early sign of necrotic cell death. See, Leist and Nicotera, 1997; Majno and Joris, 1995.
Inhibition of ATP synthesis or ATP depletion also causes neural cell swelling, blebbing and, if sufficiently severe, plasma membrane disruption and cell death. See, Jurkowitz-Alexander et al., 1993. The mechanisms of neural cell swelling associated with ATP-depletion remained incompletely characterized. See, Lomneth and Gruenstein, 1989; Juurlink et al., 1992; Rose et al., 1998.
One potential mechanism would be changes in Na+ and K+ concentration due to inhibition of the Na+/K+-ATPase pump. However, an equivalent degree of osmotic swelling induced by ouabain-mediated inhibition of the Na+/K+-ATPase pump in neural cells does not produce large depolarization, blebbing or cell death. See, Jurkowitz-Alexander et al., 1992; Brismar and Collins, 1993. Failure of the Na+/K+-ATPase pump, therefore, is not the mechanism critical to swelling of neural cells. None of these studies have identified the cellular mechanism instrumental in the cell swelling that is associated with brain damage in cerebral ischemia and traumatic brain injury and spinal cord injury.
One subtype of ATP sensitive cation channel is a non-selective cation channel, that is sensitive to Ca2+ and ATP. More specifically, some non-selective cation channels are activated by intracellular Ca2+ ([Ca2+]i) and inhibited by intracellular ATP ([ATP]i). Although Ca2+- and ATP-sensitive cation channels had been identified in a number of non-neural cell types, they have not been identified in astrocytes or any other neural cells. See, Sturgess et al., 1987; Gray and Argent, 1990; Rae et al., 1990; Champigny et al., 1991; Popp and Gogelein, 1992; Ono et al., 1994, each of which is hereby incorporated by reference in its entirety. These non-astrocyte channels comprise a heterogeneous group with incompletely defined characteristics. They exhibit single-channel conductances in the range of 25-35 pS, discriminate poorly between Na+ and K+, are impermeable to anions, for the most part impermeable to divalent cations, and they are blocked by similar concentrations of the adenine nucleotides ATP, ADP and AMP on the cytoplasmic side. The function of these non-selective ATP sensitive cation channels in these non-neural cell types remains enigmatic, in part because unphysiological concentrations of Ca2+ are generally required for channel activation.
Another subtype of ATP sensitive cation channel is the ATP-sensitive potassium channel (KATP channels) in pancreatic β cells. One class of insulin secretagogues, the antidiabetic sulfonylureas, is used to inhibit these KATP channels and stimulate insulin release in diabetes mellitus. See, Lebovitz, 1985. Antidiabetic sulfonylureas mediate their effect on KATP channels via a high affinity sulfonylurea receptor (SUR). See, Panten et. al., 1989; Aguilar-Bryan et. al., 1995. Several isoforms of the SUR, termed SUR1, SUR2A, SUR2B, and SUR2C, have been identified and cloned. See, Aguilar-Bryan et. al., 1995; Inagaki et. al., 1996; Isomoto et. al., 1996; Lawson, 2000. These receptors belong to the ATP-binding cassette (ABC) transporter family, of which the cystic fibrosis transmembrane conductance regulator (CFTR), another ion channel modulator, is also a member. See, Higgins, 1992; Aguilar-Bryan et. al., 1995. Notably, the CFTR has major therapeutic importance, since its genetic absence causes cystic fibrosis, a fatal disease.
The sulfonylurea receptor imparts sensitivity to antidiabetic sulfonylureas such as glibenclamide and tolbutamide. Also, SUR is responsible for activation of the potassium channel by a chemically diverse group of agents termed K+ channel openers (SUR-activators), such as diazoxide, pinacidil, and cromakalin. See, Aguilar-Bryan et. al., 1995; Inagaki et. al., 1996; Isomoto et. al., 1996; Nichols et. al., 1996; Shyng et. al., 1997b. In various tissues, molecularly distinct SURs are coupled to distinct channel moieties to form different KATP channels with distinguishable physiological and pharmacological characteristics. The KATP channel in pancreatic β cells is formed from SUR1 linked with a K+ channel, whereas the cardiac and smooth muscle KATP channels are formed from SUR2A and SUR2B, respectively, linked to K+ channels. See, Fujita and Kurachi, 2000.
Gliotic Capsule
The gliotic capsule that forms around a “foreign body” in the brain is an important, albeit neglected, biological system. On the one hand, the gliotic capsule represents the response of the brain to an injurious stimulus—an attempt by the brain to wall off, isolate, dispose of, and otherwise protect itself from the foreign body. On the other hand, the gliotic capsule forms a potentially harmful mass of tissue from which originates edema fluid that contributes to brain swelling, and whose constituent cells undergo cytotoxic edema, which adds further to brain swelling. Also, the gliotic capsule protects foreign cells from immunologic surveillance.
The essential elements involved in formation of a gliotic capsule appear to be uniform in many types of CNS pathology, be it a traumatically implanted foreign body, a metastatic tumor, a brain abscess, or infarcted necrotic tissue following a stroke. First, microglia and astrocytes become activated near the site of injury, with large, stellate-shaped GFAP-positive reactive astrocytes forming the most prominent cellular component of the response. Secondly, the foreign nature of the entity is recognized, and the response is initiated to surround and contain it. Although the concept of “foreign body” encompasses a large variety of pathological conditions, the responses in most cases bear a great deal of similarity to one another.
The interface between the foreign body and the gliotic capsule, referred to as the inner zone of the gliotic capsule, appears to be of great importance in determining the overall response to injury.
Thus, a need exists for a physiological target instrumental in the cell swelling that is associated with brain damage in cerebral ischemia and traumatic brain injury and in the consequent morbidity and mortality. There is also a need for specific treatments for the cytotoxic edema that causes brain swelling, which worsens outcome and increases morbidity and mortality in brain injury and stroke. Other and further objects, features, and advantages will be apparent from the following description of the presently preferred embodiments of the invention, which are given for the purpose of disclosure.